As is known in the art, one way to improve performance and reduce the size of power electronics is through increasing switching frequency. Resonant dc/dc power converters enable higher switching frequencies than can be achieved with conventional pulse-width modulated circuits, due to their natural soft-switched operation and ability to absorb and utilize circuit parasitics in the conversion process. For example, efficient resonant dc/dc power conversion has been demonstrated at frequencies in excess of 100 MHz, and operation at much higher switching frequencies is feasible.
A limitation of known resonant converter circuits is the sensitivity of the inverter stage to loading conditions. Switched-mode radio-frequency (rf) inverters suitable for ultra-high frequencies (e.g., classes DE, E, and F) exhibit high sensitivity to the effective impedance of the load. For example, class E inverters only operate under soft-switched conditions over about a factor of two in load resistance. While acceptable in communications applications (in which the load resistance is relatively constant), this is problematic for many dc/dc power converter applications, where the effective resistance presented by the matching stage and rectifier varies greatly with output voltage and current. This problem is particularly severe in applications in which the voltage conversion ratio varies substantially; such applications include charging systems where the converter must deliver constant power over a wide output voltage range and regulating converters where the converter must operate over a wide input voltage range and/or the same converter design must be capable of supporting a range of output voltages.